(i) Field of the Invention
This invention relates to a polymeric insulating material having excellent electrical characteristics. Specifically, the present invention is concerned with a polymeric insulating material having good electrical insulating property and made of polypropylene of particular physical properties and also with a formed article making use of the same. Notably, the present invention pertains to a polymeric insulating material having excellent electrical characteristics even when formed thin and also to a formed article of a small thickness obtained from the material.
(ii) Description of the Related Art
Polymeric materials such as polyolefins, polyesters, polyfluorinated vinylidene, silicone resins, epoxy resins and polystyrene generally have a low electrical conductivity and are used as a wide variety of electrical materials by making use of electrical characteristics such as insulating property. In addition to the excellent electrical property, these polymeric materials also feature high flexibility, heat resistance and hydrophobicity so that they have found a wide spread utility in many products.
Polyolefins, especially polypropylene can be easily molded or otherwise formed or processed, possess superb electrical, mechanical and chemical properties and, owing to their availability at low price, are extensively employed as various electrical materials. They are widely used as electrical insulating materials, for example, as electret materials in the form of filters or films, as electrical insulating tapes or covering materials, or as capacitor insulating films in the form of biaxially-oriented films. As propylene employed for such products, the higher the insulation resistance, the better. Higher dielectric breakdown strength furnishes products of higher performance. Improvements have hence been made in this respect.
Insulation characteristics of polypropylene employed these days are however not considered very good. Production of polypropylene having still better insulation characteristics, if feasible, will be extremely meritorious from the industrial viewpoint. Toward this goal, high purification of polypropylene has therefore been studied to date. For example, Japanese Patent Laid-Open Nos. 110906/1986, 63609/1984, 188627/1983, etc. disclose that the dielectric breakdown strength of biaxially-oriented polypropylene can be enhanced by increasing the stereoregularity of starting polypropylene, reducing the content of boiling n-heptane soluble matter and hence increasing the crystallization degree. Further, Japanese Patent Laid-Open Nos. 113548/1987, 254749/1989, 166955/1989, 150443/1990, etc. disclose that insulation characteristics of polypropylene can be improved by lowering as much as possible the contents of catalyst residue or chlorine remaining in the polypropylene.
Industrial production of high-purity polypropylene so improved, however, results in the problem of high production cost. An increase in stereo-regularity leads to a higher crystallization degree, which in turn results in the problems that formed products tend to become harder and the formability or processability is deteriorated. Upon spinning, for example, it is difficult to spin such polypropylene, and when stretched, the polypropylene may undergo stretching breakage. When formed into a film, voids may be formed. As a consequence, such increased stereoregularity can provide, as products, only formed products having poor insulation characteristics. In practice, it is very difficult to completely eliminate impurities so that impurities as residue from a catalyst and the like are contained more or less.
There is an extremely strong demand for the dimensional reduction of electrical and electronic parts. In such electrical and electronic parts, polypropylene is used as an insulating material such as capacitor films. To meet the above demand, it is desired to increase the breakdown resistance of the insulating material so that the insulating material can be reduced in thickness to have smaller dimensions.
Stretched polypropylene films obtained by such conventional methods have rather high dielectric breakdown resistance where they have a thickness of approximately 10 .mu.m or greater but, when their thicknesses are reduced to about 6 .mu.m or less, especially about 4 .mu.m or smaller, develop the drawback that the dielectric breakdown resistance per unit thickness is extremely decreased. It is therefore difficult to obtain thin films having high dielectric breakdown resistance, thereby still failing to meet the demand for the dimensional reduction of electric and electronic parts.